Water in Homestake drops to the 4850 Level
After more than a year of continuous effort at the former Homestake gold mine in Lead, South Dakota, on 13 May pumps brought the water level down to below the historic 4850 ft level (1478 m underground). This event paves the way to re-open the site of the world's first solar-neutrino detector. Ray Davis began the original experiment in 1965; it ran for almost three decades and brought him a share of the 2002 Nobel Prize in Physics (CERN Courier December 2002 p15).
The State of South Dakota is investing $121 million to convert Homestake into a dedicated science and education centre – initially through the activities of the Sanford Underground Laboratory at Homestake, which plans a major experiment in the same cavern used by Davis. This will be followed by a proposed investment of $550 million for the national Deep Underground Science and Engineering Laboratory (DUSEL), currently being developed by the US National Science Foundation (NSF). DUSEL is set to become the largest, deepest underground laboratory in the world and will more than double the world's current underground-laboratory space (CERN Courier September 2007 p9).
When the Homestake Mining Company announced in September 2000 that the 125-year-old mine would shut down, scientists were quick to suggest converting it into a multidisciplinary underground laboratory for physics, geology, hydrology and biology. Progress was slow, however, and when the owners turned off the pumps in 2003 the water level gradually rose to 1380 m below ground.
It took a $10 million federal grant, $41 million from the South Dakota Legislature and a $70 million donation from local philanthropist, T Denny Sanford, to get the pumps started again in 2008, under the auspices of the South Dakota Science and Technology Authority (SDSTA). They will continue to pump until they reach Homestake's deepest level, 2500 m below ground. Several experiments are planned at various depths in the cosmic-ray-shielded, low-background-radiation environment.
The SDSTA has already begun to refurbish the "4850 Level" and clean out the Davis cavern for the Sanford laboratory's new dark-matter experiment, the Large Underground Xenon (LUX) detector. Other experiments planned for the 4850 Level, which will be one of DUSEL's main "campuses", include the search for neutrinoless double-beta decay – indicating that neutrinos are their own antiparticles – and a high-current, low-energy ion accelerator to study astrophysical nuclear reactions. Some of these experiments anticipate additional installations at the laboratory's 7400 Level (2256 m underground), which offers added shielding from backgrounds.
Planning for DUSEL is being led by the University of California, Berkeley, and the Lawrence Berkeley National Laboratory (LBNL), in collaboration with other institutions including the South Dakota School of Mines & Technology (SDMST) and Black Hills State University. Kevin Lesko, of Berkeley and LBNL, and Bill Roggenthen, of SDMST, are the project's joint principal investigators. "Regaining access to the 4850 Level is critical for DUSEL," says Lesko. "It allows the re-introduction of physics experiments into Homestake – the birthplace of neutrino astrophysics." Funding for the construction of DUSEL could come as early as 2012.
The biggest experiment intended for the 4850 Level is the Long Baseline Neutrino Experiment proposed by the US Department of Energy. For this the NSF plans to excavate one or more vast cavities, each holding 100 000 tonnes of ultrapure water and large enough to house nearby Mount Rushmore whole. Fermilab, 1300 km to the east of Homestake, would generate the neutrino beam aimed at the huge detectors, while the Brookhaven National Laboratory would build the instrumentation. In March this year the governor of South Dakota, Mike Rounds, and representatives of SDSTA, NSF, and DUSEL – including Lesko and Roggenthen – welcomed the laboratory directors, Pier Oddone of Fermilab and Sam Aronson of Brookhaven for a tour of the Sanford Laboratory.
Meanwhile, research has already begun at the Sanford Underground Laboratory. Roggenthen, a geologist, has installed a 3D seismic array with sensors at several levels. Preliminary experiments are also under way in biology and hydrology, and the mine is being surveyed for a gravity-wave detector. Members of the LUX collaboration will install their dark-matter experiment later this year.